Fiber composite materials, also known as prepregs, are composite materials having a reinforcement fiber that is pre-impregnated with a thermoplastic or thermoset resin matrix. The resin holds the fibers together and enables formation of unidirectional fibers or fabric. The amount of resin within the matrix is generally the amount desired in the final composite material so that no additional resin need be applied during manufacturing, hence the name “prepreg.” The resin matrix in a prepreg is usually partially cured for ease of handling. Sheets of prepreg are provided with silicone-coated paper on each side and sold in a roll format. The prepreg material is applied to an article then heated, for example in an autoclave, to fully cure and polymerize the resin in the desired shape composite article. The prepreg rolls are stored in a freezer, generally at 0° F. for up to six-months, to prevent premature polymerization or curing during storage.
Manufacturing composite materials requires thawing the prepreg after removal from the freezer for a set period of time. Thawing of such a material can take several hours, sometimes upwards of 16 hours. The amount of time a prepreg material spends at room temperature is critical to its shelf-life. When a prepreg roll is removed from the freezer, the amount of time it is out of the freezer must be carefully noted. This is generally referred to as the roll's “out-time.” If the entire roll is not consumed during one out-time, the remainder be returned to the freezer to preserve the material's shelf life.
As manufacturing volume increases, the present inventors have found several problems with conventional prepreg roll systems resulting in either downtime of the manufacturing line or unacceptable waste due to out of time material. When working through a roll of prepreg, it is critical to know exactly how much material is left on the roll to determine whether removing the next roll from the freezer will be necessary to complete a job. Operators will begin to thaw the next roll if they estimate they will need additional material to complete a project. In practice, this requires keeping track of exactly how much material has been removed from a roll and the rate at which it is being removed.
If the operator underestimates the amount of material left on the roll, and begins unnecessarily thawing a new roll, that material could exceeding its out-time and is therefore wasted. If the operator overestimates the amount of material left on the roll, the entire process might shut down while awaiting the next roll begins to thaw. This can also result in problems with uncured material present on a partially built workpiece. If the uncured prepreg goes out-time, then the partially built uncured part could be sacrificed.
In order to avoid out-time prepreg, operators base their estimate the amount of remaining material by keeping track of the length of plies cut from a roll of material. These estimates often contain rounding error, gauge error, and typographical errors in maintaining the amount of material cut. The result is that preparation, including thawing, of the new roll begins too early or too late and the corresponding wasted material or process downtime as discussed above. Although these methods have worked for lower volume manufacturing operations, as throughput has increased the need systems that better monitor and control the process are needed.